Installing GPG Tools on Mac sets up gpg-agent automatically and has Mac Keychain support. I use the agent support all the time for signing Maven artifacts (which requires password-less entry due to the way the release plugin works).
This isn't yet at S/MIME levels, much less something like iMessage or Signal/TextSecure – and, yes, there's a ton of room to argue their respective merits but most users are simply going to switch to something which gets their message through rather than debug failures, particularly when the software doesn't give any feedback about what's wrong and, critically, what you could do to fix it.